'Oumuamua was
just passing through the solar system, with so much extra
velocity (about 25 km/sec) that it was moving faster than the
Sun's escape velocity.

This was a one-time
visitor, screaming through the solar system and heading back out
into The Black once again.

That was certainly
enough to make it the object of intense scrutiny.

We'd never seen
something from interstellar space pass through the solar system
before! But what was it?

At first it was
classified as a comet, then an asteroid, and
then maybe a comet again (this confusion is reflected in its
provisional designations; at first it was A/2017 U1, for "asteroid",
then C/2017 U1, for "comet", then finally I/2017 U1, for
"interstellar").

It was hard to tell
what it was:

it was too small, faint, and far away to get
good observations, and worse, it was only seen on its way out, so it
was farther from us literally every day.

As it moves away,
the Sun's gravity pulls on it, slowing it down… but it wasn't
slowing down enough.

Some force was
acting on it, accelerating it very slightly. Comets are made of rock
and ice, so maybe the ice was turning into gas, and as this was
blown off it acted like a very gentle rocket. The problem with this
is that no such venting was detected.

If it were like
comets in our solar system, you'd expect to see lots of carbon
monoxide (CO) and carbon dioxide (CO2) coming from it,
but none was seen.

So maybe it was
some other kind of ice, like water.

But again, if it is
like our local comets, it would take so much water that we'd have
noticed.

Maybe this
force was radiation pressure, literally the force of sunlight
hitting it and giving it a tiny push.

That makes some
sense, but for the math to work out with the acceleration seen, 'Oumuamua
had to be flat.

Like, really flat:

So thin that it
looked more like a solar sail, a very thin sheet of material
designed to catch sunlight and accelerate.

But that, in turn,
meant that 'Oumuamua was artificial. As in, a spaceship...

Besides the obvious
(it seems like a big leap!),
I have my problems with this idea. Not much has changed with
that hypothesis since I wrote that, and while I wouldn't dismiss it
being an alien probe out of hand, the evidence doesn't support that
conclusion, and in fact points against it.

That's because the
amount of pressure sunlight exerts is very small, so if an object is
massive it has to be spread out very thin and big to catch enough
sunlight to accelerate it enough to match the observations.

So they assumed it
had some normal density like 1-3 grams per cubic centimeter (roughly
somewhere between the density of water to rock).

The new paper turns
that around.

Instead of assuming a density to find the area, let's
assume the size determined
using normal methods is correct, use that to get an area, and
from there get the density needed to match the observations.

Assuming a size for
'Oumuamua of 50-130 meters, what they get is a very low
density:

About 0.00005 grams per cc.

That's incredibly
low, and at first it seems ridiculously so. That's 100 times less
dense than air! No solid object could have a density that low!

…so, what if it's
not solid?

I'm not saying it's
hollow, but maybe it's really porous. Like Styrofoam or Swiss
cheese, but much much holey-er. Is there anything natural like that?

Materials made in a
fractal pattern can be very porous, and in fact out in that
protoplanetary disk around a young star, physical models show that
objects can grow fractally until they're as big as 'Oumuamua, and
have those extremely low densities needed to account for its weird
behavior.

So 'Oumuamua
doesn't have to be a spaceship. It just has to be a snowflake! A
three-dimensionally constructed phenomenally porous low-density
snowflake.

And in fact this
actually makes sense.

The fact we see
something like 'Oumuamua coming from interstellar space means we can
try to estimate how many of them are out there at any time.

Making some simple
assumptions, the numbers you get
are way too high to be explained by having the galaxy filled
with such beasts. It seems far more likely that objects like 'Oumuamua
are relatively rare, and that means it likely came from someplace
close by (if it came from farther away, the odds are even lower we'd
ever see one).

Working out the
math, the new paper suggests it came from a nearby star, and one
that's relatively young (less than 100 million years). It formed out
in the disk, and got ejected somehow, likely from a planet forming
nearby giving it a boost from its gravity.

I have to say, I
love this. It may not be as sexy or headline-grabbing as saying it's
an alien probe, but it's very clever, it uses known materials and
math, and the conclusion even fits in with what we know about how
planets form.

That makes it
pretty dang cool in its own right. If I had to bet, I'd put a lot
more money on this idea being correct over 'Oumuamua being an
artifact from another civilization.

It may be we'll
never know; it's moving pretty rapidly relative to us, and is very
far away now - it's currently over 1.6 billion kilometers away.
In a year it'll be 900 million km further. So it's not like we
can catch up to it anytime soon.

We'll just have to
wait for another such object… which, according to this new idea, may
take a long time.

But we're scanning
the skies with better tech, seeing deeper, seeing fainter objects,
all the time now. I certainly hope we find more beasties like this
one.

They can tell us so
much about how planets form in other star systems, which is pretty
hard to figure out from dozens or hundreds of light years away.

It's a lot easier
when they obligingly send bits of their building materials to us.